Ethylene and propylene, butenes, butadiene and isoprene are obtained, at present, by the pyrolysis, in the presence of water vapor, of lower paraffins (ethane, propane, butane) or of low boiling hydrocarbon cuts such as straight run gasoline with reduced content of aromatic hydrocarbons.
The shortage of these raw materials in the past few years has made it desirable to use the heavy cuts of hydrocarbons, namely gas oils, vacuum distillates or crude oil as the pyrolysis feedstock.
As shown in Table 1, depending on the oil cut used, the yields obtained are different. (M.I. Offer, Hydrocarbon Processing 55 No. 10, Oct. 1976, p. 123).
TABLE 1 ______________________________________ Distribution of products from cracking of the hydrocarbons cuts with water vapor. Cut Distilling range .degree.C. Gasoline Gas oil Gas oil Pyrolysis 32-171 DA 232-327 DAV 343-546 Severity Low High Low High Low High ______________________________________ CH.sub.4 10.3 15.0 8.0 13.7 6.0 9.1 C.sub.2 H.sub.4 25.8 31.3 19.5 26.0 16.0 19.0 C.sub.2 H.sub.6 3.3 3.4 3.3 3.0 3.2 3.8 C.sub.3 H.sub.6 16.0 12.1 14.0 9.0 12.5 12.2 1.3-C.sub.4 H.sub.6 4.5 4.2 4.5 4.2 4.5 4.9 Other C.sub.4 hydro- carbons 7.9 2.8 6.4 2.0 5.4 3.4 C.sub.5 cut-205.degree. C. 27.0 22.0 20.0 20.0 14.0 11.0 Fuel Oil 3.0 6.0 21.0 19.0 31.0 32.0 Other components 2.2 3.2 3.3 3.1 7.0 4.6 ______________________________________
Crude oil cracking in the presence of water vapor at very high temperatures (Japan Pat. No. 48-10763.1973) was recently tested on an industrial scale. The results obtained are given in Table 2.
TABLE 2 ______________________________________ Crude oil pyrolysis by Kureha-Union Carbide process. Crude oil from Feed Syria Minas Aramco Kafuji 0 1 2 3 4 ______________________________________ Specific weight (g/cm.sup.2) 0.840 0.844 0.853 0.880 Gasoline up to 200.degree. 40 16 32 27 Oil (220.degree.-250.degree. C.) % wt. 20 8 12 8 Gas oil (250.degree.-350.degree. C.) % wt. 15 10 10 10 Waste oil % wt. 25 66 46 55 Sulphur 0.07 0.07 1.72 2.92 Working conditions: Steam temperature (.degree. C.) 1650 1450 2000 1650 Gravimetric ratio of water to crude oil 2.6 3.2 2.8 2.6 Reaction time (seconds) 0.01 0.01 0.005 0.01 Products (% wt.) CH.sub.4 7.7 5.1 13.7 7.8 C.sub.2 H.sub.2 1.4 1.1 17.7 1.6 C.sub.2 H.sub.4 26.2 29.4 18.6 25.2 C.sub.3 H.sub.6 9.5 11.2 0.7 8.7 C.sub.4 H.sub.6 3.4 3.7 0.5 2.8 H.sub.2 + C.sub.3 -C.sub.4 cut 12.8 5.5 8.8 7.9 Benzene cut; boiling temp. cut 200.degree. C. 2 1 5 1 Naphthene cut; boiling temp. cut 200.degree.-250.degree. C. 12 11 4 10 Heavy cut 250.degree.-450.degree. C. 15 20 6 15 Waste cut; boiling temp. cut &gt; 450.degree. C. 10 12 25 20 ______________________________________
The disadvantages of vapor cracking of heavy crude oil cuts are mainly the large amount of waste fuel and from the relatively low yields of C.sub.2 -C.sub.4 olefins. Improvements in this field can be obtained if the process is performed in the presence of thermal carriers (coke, silica, refractary oxides) preferably in a fluidized bed or mobile bed of a thermal carrier. A synthesis of these processes is described by R. Chelle and G. Henrich (Chimie et Industrie, Genie Chimique Vol. 104, No. 4-5 March 1971, p. 413-421).
Up to the present these processes have not been developed on industrial scale due to the fact they require high reaction temperatures within the operation range of cracking units with water vapor, the mechanical corrosion of reactors is an important deterrent.
Recently, starting from these processes, the catalytic processes of hydrocarbon pyrolysis have been tried.
According to West German Pat. No. 1,927,061, the pyrolysis reaction is achieved on catalysts consisting of Mg and Zr oxide mixtures, with lanthanide admixtures, Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3, SiO.sub.2 or Ca, Mg, Sr and Ba oxides. Alternative catalysts based on refractory metal oxide are described in the following patents: West German Pat. No. 1,971,400, England Pat. No. 1,369,242, France Pat. No. 1,603,019, U.S. Pat. Nos. 3,725,495 and 3,767,567, Japan Pat. No. 7,504,002. Also according to the Soviet Union Pat. No. 410,073, the use of potassium niobiate as a catalyst has the advantage of reducing the coke evolved in the process. At present niobium is not available in large amounts for such use.
All these catalysts have the disadvantage that they work at high temperatures, up to 900.degree. C., within the range of thermal processes.
Hence it is difficult to emphasize the catalytic contribution to the pyrolysis process and to estimate the difference between these processes and the process with thermal carriers.
A catalyst working at relatively low temperatures (500.degree.-700.degree. C.) consisting of fluorinated alumina is claimed in the West German Pat. No. 1,277,842. Hydrocarbon cuts from C.sub.4 to cuts having boiling temperatures of 450.degree. C. can be used as the pyrolysis feedstock.
It is a specific object of this process to obtain propylene as the main product, the ratio of propylene to ethylene being higher than 1 and it provides a high content of aromatic hydrocarbons in the liquid cut from the pyrolysis. The low ethylene yields cannot be improved by recycling the liquid pyrolysis cut due to its aromatic nature. Fluorine also causes instability and corrosion in the apparatus.
The use of new types of catalysts, completely cationized molecular sieves with a ratio of SiO.sub.2 to Al.sub.2 O.sub.3 within the range 3 and 20 is described in the U.S. Pat. No. 3,647,682. The relative low thermal stability of molecular sieves makes it possible to use them at relatively low temperatures and to obtain mainly propylene and saturated C.sub.2 and C.sub.3 hydrocarbons.